1. Field of the Invention
The present invention relates to communications networks in general and in particular to PLL for recovering data and clock signals from encoded electrical signals.
2. Prior Art
The prior art abounds with Phase Lock Loops (PLLs) which are used to recover clock and/or data embedded in run length code data signals. The PLLs may be used in recording devices to recover prerecorded information (data and/or clock) from a recording medium or in Data Terminal Equipments (DTES) to recover information transmitted over the transmission medium of a communications network.
A search of the prior art discloses the below-listed references. The first four references relate to PLL used in recording devices; while the remaining references relate to PLL used in DTEs.
U.S. Pat. No. 4,583,053 to Yarborough PA1 U.S. Pat. No. 4,633,488 to Shaw PA1 U.S. Pat. No. 4,737,866 to Ebata PA1 U.S. Pat. No. 4,857,866 to Tateishi PA1 U.S. Pat. No. 4,750,193 to Bailey PA1 U.S. Pat. No. 4,803,705 to Gillingham et al PA1 U.S. Pat. No. 4,847,874 to Kroeger et al PA1 U.S. Pat. No. 4,908,841 to Leis et al PA1 IBM.RTM. TDB, Vol. 27, No. 12, May 1985 (pgs. 6988-6991).
The prior art suggests that a conventional PLL has three major functional units. The units are an oscillator (voltage or current controlled), a loop filter and a phase detector. The phase detector (PD) compares the output signals from the oscillator with input reference signals and outputs DC output signals proportional to the phase difference between compared signals. The DC output signals are passed through the loop filter to produce an average error signal which is used to control the frequency of the oscillator.
Several problems and solutions to them have been identified with the conventional PLL structure. Broadly classified, the problems include difficulty in locking the free running frequency of the oscillator to the frequency of the input reference signals, locking to harmonics or beat frequencies and locking within an optimum time period. The prior art has added a variety of electrical circuit arrangements to overcome specific identified problems. In some cases the identified problems are solved by simply rearranging the functional units and/or optimizing the components used in the units.
Even though the prior art addresses (i.e., identify and provide solutions) several of the problems associated with PLLs, none of them addresses the problem of variable gain. It is believed that variable gain is a major cause of instability in PLLs. Similar to other instability factors, variable gain causes jitter in the incoming signals. The jitter, in turn, makes it difficult to extract clocking signals from incoming signals and use of the clocking signals to synchronize the data.
The variable gain problem is particularly destructive to local area networks (LANs) such as the type espoused by IEEE 802.5, popularly called the token ring network. This type of LAN is well known in the prior art, therefore, only those features which make the LAN susceptible or sensitive to variable gain problems will be discussed further.
The token ring LAN is a serial network in which a large number of DTEs (also called stations) are connected to a looped transmission medium. Signal transmission is effectuated serially on the transmission medium. Thus, an active station (n) receives bits of a frame in a sequential fashion from its nearest active upstream neighbor (n-1) and retransmits the bits to its nearest active downstream neighbor (n+1). Because of the large number of stations sending (transmitting) and/or receiving frames from a common transmission medium any problem (such as loop gain variation, etc.) in one station accumulates and accentuates instability in the overall network. The problem is even more severe where the stations are provided by different manufacturers. Each manufacturer may set different loop gain variation standards which may not cause severe problems if all stations on the same loop are from the same manufacturer. However, by mixing stations from different manufacturers, the different standards could result in unacceptable loop gain variations.
The scheme used to encode data is another feature which makes the token ring LAN susceptible to variable loop gain. The well-known Manchester signalling technique is used to encode data on the token ring network. This signalling technique is selected because of its efficiency in transmitting both data and timing information in the same signal. In Manchester signalling techniques, different bit patterns are used to convey different types of information. It is believed that the variations in the data pattern also causes variation in the loop gain.